Circadian or daily rhythms modulate physiological responses. Robust daily rhythms are predictive of improved prognosis for cancer patients, independent of performance status measures. Disruption of circadian rhythms is associated with poor sleep quality and negative mood, fatigue, and reduced quality of life. The ability to care for a patient at home is often lost when the patient no longer sleeps during the night. The mechanism by which tumors suppress circadian rhythms and impair quality of life is unknown. We hypothesize that cytokine release induced by tumors may act directly in the neural system driving circadian rhythms and this action may induce fatigue and circadian rhythm disruption. Previous studies have shown that central administration of the cytokines TGF-1 and neuregulin-1 and systemic administration of IFN-1 can disrupt behavioral rhythms in hamsters and mice. As an animal model of tumor-induced disruption, we will determine if peripheral administration of TGF-1 and neuregulin-1 can similarly disrupt locomotor activity rhythms in mice. We will determine if cytokines can suppress the rhythms expressed by isolated suprachiasmatic nucleus (SCN), immune system organs, thymus and spleen, and other tissues (lung and mammary gland). Further experiments will assess if erlotinib or gefitinib, blockers of epidermal growth factor receptor (EGFR) tyrosine kinase activation used for chemotherapy, can improve circadian rhythm regularity and amplitude following disruption by TGF-1. These experiments are designed to mimic clinical data showing such effects in patients. We will determine with animals the optimal timing for administration of erlotinib or gefitinib to maximize the potential benefit to the circadian system. This research will increase our understanding of the biological mechanism by which tumor growth can impact the circadian system. We will describe effects of circadian clock output suppression on immune system function and on rhythms endogenous to select organs. Our studies will clarify if such effects could be mediated by cytokines, if action is peripheral or central, and if clinical reports of improved quality of life following administration of specific cytokine receptor blockers might be attributable to improved circadian regularity. This research will apply directly to clinical use of EGFR blockers, by investigating a rationale for optimal timing of these compounds. Cancer patients often suffer from disrupted circadian rhythms resulting in poor sleep quality, negative mood, fatigue and reduced quality of life. Tumors induce cytokine release, which may be responsible for circadian disruption and subsequent fatigue. By blocking the action of cytokines with specific compounds, we hope to clarify the downstream effects of cytokines on the immune and circadian systems. Clinical reports of improved quality of life after administration of cytokine blockers might be attributable to improved circadian rhythms.